Vital signs of a person, for example the heart rate (HR) or respiratory information (respiratory parameters) such as the respiratory rate (RR), can serve as a powerful predictor of serious medical events. For this reason the respiratory rate and/or the heart rate are often monitored online in intensive care units or in daily spot checks in the general ward of a hospital. Besides the heart rate, the respiratory rate is one of the most important vital signs. Both, the HR and the RR are still difficult to measure without having direct body contact. In present intensive care units, thorax impedance plethysmography or the respiratory inductive plethysmography are still the methods of choice for measuring the RR, wherein typically two breathing bands are used in order to distinguish thorax and abdominal breathing motion of a person. The HR is typically measured by use of electrodes, fixed at the chest of the subject, wherein the electrodes are connected to remote devices through cables. However, these obtrusive methods are uncomfortable and unpleasant for the patient being observed.
Moreover, unobtrusive respiratory rate measurements can be accomplished optically by use of a stationary video camera. A video camera captures the breathing movements of a patient's chest in a stream of images. The breathing movements lead to a temporal modulation of certain image features, wherein the frequency of the modulation corresponds to the respiratory rate of the patient monitored. Examples of such image features are the average amplitude in a spatial region of interest located around the patient's chest, or the location of the maximum of the spatial cross correlation of the region of interest in subsequent images.
Further, one or more video cameras are used for unobtrusively monitoring the HR, the RR or other vital signs of a subject by use of remote photoplethysmographic imaging. Remote photoplethysmographic imaging is, for instance, described in Wim Verkruysse, Lars O. Svaasand, and J. Stuart Nelson, “Remote plethysmographic imaging using ambient light”, Optics Express, Vol. 16, No. 26, December 2008. It is based on the principle that temporal variations in blood volume in the skin lead to variations in light absorptions by the skin. Such variations can be registered by a video camera that takes images of a skin area, e.g. the face, while the pixel average over a selected region (typically part of the cheek in this system) is calculated. By looking at periodic variations of this average signal, the heart rate and respiratory rate can be extracted. There are meanwhile a number of further publications and patent applications that describe details of devices and methods for obtaining vital signs of a patient by use of remote PPG.
Thus, the pulsation of arterial blood causes changes in light absorption. Those changes observed with a photodetector (or an array of photodetectors) form a PPG (photo-plethysmography) signal (also called, among other, a pleth wave). Pulsation of the blood is caused by the beating heart, i.e. peaks in the PPG signal correspond to the individual beats of the heart. Therefore, a PPG signal is a heart rate signal in itself. The normalized amplitude of this signal is different for different wavelengths, and for some wavelengths it is also a function of blood oxygenation or other substances found in blood or tissue.
With respect to camera based systems, a superposition of vital sign signals, such as a respiratory rate signal superimposed by a heart rate signal or vice versa, adversely affect the determination of respiratory information. Such a superposition of vital signs can e.g. be measured, when a camera system observes the thorax motion of a subject, wherein the thorax motion due to breathing is superimposed by movements related to heart rate signals, so-called cardiac seismograms. These superimposed signals can have a comparable magnitude and even a comparable frequency. This might lead to dangerous situations, in particular during a period without breathing. Errors can occur that are related to the superimposed heart rate signal, which could give the impression that a respiratory rate is detected, in which an apnoea phase is present.
Episodes of apnoea often appear in premature babies as well as in adult population suffering from sleep related diseases. The common manner to monitor apnoea consists in placing contact sensors, in a rather obtrusive way, to the patient. These ranges from nasal thermocouples, respiratory effort belt transducer, piezoelectric transducer, optical sensor and electrocardiography ECG. Next to being really obtrusive these sensors are also prone to wrong measurement as some implementation cannot distinguish breathing from motion and most require calibration.
Jae Hyuk Shin et al.: “Nonconstrained Sleep Monitoring System and Algorithms using Air-Mattress with Balancing Tube Method”, IEEE Transactions on information technology in biomedicine, IEEE Service Center, Los Alamitos, Calif., US, vol. 14, no. 1, 1 Jan. 2010 (2010-01-01), pages 147-156 discloses a bed-type sensor system using the air-mattress with balancing tube (AMBT) method to non-invasively monitor the signals of heartbeat, respiration, and events of snoring, sleep apnea and body movement of subject on the system. The proposed system consists of multiple cylindrical air cells, two sensor cells and 18 support cells. Small physiological signals are measured by the changes in pressure difference between the sensor cells, and the DC component was removed by balancing tube that is connecting the sensor cells. Using the AMBT method heartbeat, respiration, snoring, and body movement signals were clearly measured.
EP 0 072 601 A1 discloses a respiration monitor and x-ray trigger apparatus wherein analog respiration signals are amplified and divided into respiratory and heartbeat components. A trigger signal is selectively generated to the x-ray machine just prior to occurrence of a selected respiration extrema. Apnea alarms are generated if a predetermined decrease in respiration rate is detected in successive periods in conjunction with a predetermined number of decelerating heartbeats. An alarm is also generated if the respiration and heart rates are equal for a predetermined number of periods or if the respiration or heart rates stray outside of preset threshold values.